INDUCED DISEQUILIBRIUM AND THE RADIOMETRIC ASSAY OF PERCUSSION BOREHOLE PULPS

One of the most cost-effective methods for the prospecting and evaluation of a valley-fill surficial uranium deposit is to drill it by means of a percussion drill [2]. Simplistically, the hole is drilled dry using compressed airto drive the drill and also to blow the chips from the hole up to the surface, where they are collected by a suitable method, usually a cyclone The sample is collected m a bag at the bottom of the cyclone and a small, fine-grained, but commonly high-grade portion, is blown away as dust. The geologist may then radiometncally log the powder m the bags on site a short time (e.g. 0.5 to 3 h) after the sample has been taken, or alternatively m a storage area after a few days In this way, a rapid continuous "grade control" program is conducted, the results of which could mislead the geologist and cause errors in the calculation of the resources.

By the nature of the drilling operation, compressed air is forced into the hole and blows out the pulp with the simultaneous loss of 222Rn. This causes severe disequilibrium between the gamma-emitting isotopes 214Pb(t^=

26.8 m in) and 214Bi (tyi= 19.8 m in) and their longer-lived parent 226Ra (tH= 1 600 y). Because of the very short half-lives of 214Pb and 214Bi, as compared to the parent isotope, 222Rn, they will decay very rapidly without significant replenishment. The theory behind this phenomenon can be described in the following way:

The gamma-emitting isotope that is measured with a spectrometer is usually the 1.76 MeV line of 214Bi, the daughter isotope of 214Pb, 222Rn and 226Ra. As 214Pb is the immediate parent of 214Bi and the daughter of 222Rn, it can be regarded as the rate-control I ing isotope for the growth and decay of 214Bi In Figure 6 the decay curve for

214Pb and the growth curve for 222Rn are shown After about 1.5 h, about 10 % of the 214Pb remains because of its short half-life. Simultaneously, the growth of its parent ²²²Rn is negligible after the same period. Therefore, the intensity of the emitted gamma rays will decrease proportionately so that a sample can be falsely rejected, as it appears to contain a low uranium grade due to the analysis being based on incorrect assumptions inherent in the technique of field gamma-ray spectrometry.

Nevertheless, the field application of a gamma-ray spectrometer is a very useful and versatile technique if the operator is aware of the pitfalls. On site grade control is essential to the exploration geologist if he is to maintain an efficient prospecting program This is possible, all things being equal, if the necessary procedures are followed and certain corrections applied.

Assume that during the percussion drilling operation 70 % of the free radon (222Rn) gas is blown away and that the remaining 30 % is retained m the sample. That proportion of 214Pb, unsupported by the 70 % radon that was lost, will decay as shown in Figure 7, but the 30 % fraction, supported by the remaining radon, will continue to emit gamma rays Accordingly, the activity of the retained 222Rn and the supported 214Pb will be m equilibrium, but the unsupported 214Pb will decay without replenishment. The combined activity of the system, for up to a period of 3 5 h, will follow the curves as shown in Figure 7forvanousradon losses, and assumes an initial grade of uranium of 1 kg eLI/t After a decay period of three hours, the unsupported 214Pb disappears and the gamma activity of the sample will asymptotically approach an equilibrium condition where the curve becomes flat After extrapolation of this flat portion of the curve horizontally to "time zero" (i e zero decay), the relative percentage of radon retained in the ore can be estimated on the vertical scale.

In practice, this phenomenon can be used by geologists to compensate for the radon loss during percussion drilling, thereby enabling them to determine the true grade of uranium with a greater degree of confidence. It can be suitably applied to surficial uranium deposits where radon is easily lost because of its inherently high escape/production ratio.

The degree of radon loss will depend on many factors, for example, the pressure of air from the compressor, the friability and porosity of the ore, the mineralogy and radon release tempo of the uranium minerals, etc. Therefore each geological environment will have its own radon escape characteristics.

A practical example using this technique can be explained m the following way:

It is important to know the time that the sample was percussion drilled. Usually, the sample collected is representative of a 0.5 to 1 m depth section and the time at which this section was drilled (not finally collected) is regarded as "time zero' It is from this point in time that the unsupported 214Pb will start to decay. The geologist will select a few samples with different lithologies and for the next three hours measure the activity of each sample about every 1 5 mm, using fixed sample and instrument geometry in a low-background area. It is assumed that the spectrometer is calibrated, and the value obtained will have the units kg eU/t. These data are plotted onto a graph as shown m Figure 8, where the function "time lapsed after percussion drilling of sample" is the difference in time between "time zero" and the actual time that the "grade" of the samples was determined. A curve drawn through the plotted points represents the decrease of radioactivity (resulting from the decay of214Pb) and, when extrapolated to time zero, the true grade of the sample can be determined. In this example it is 0.72 kg eU/t (curve 1)

100

Decay curve for 214Pb

Growth curve for 222Rn O S

Decay and growth curves for ²'⁴Pb and ²²²Rn respectively.

P

£

°

TIME LAPSED AFTER DRILLING OF SAMPLE

Figure 7

Family of decay curves for ²¹⁴pB for surficial ore that has been partially degassed of ²²²Rn to varying degrees.

90 O tC. 80 UJ 9^70 UJtr

O 60

O Z

g 5

ceuj

H UJ

- Q l O

• f E o s

oU J 0 8 OL W Q0 7 UJ

LU 0 1

Q ' oco

• Values determined in field with a spectrometer

\ Extrapolated values of actual equivalent grade of uranium at time zero {= 0 72 Kg e U/t) X Normalized values from Curve 1

, , | , , , , | , , , . I , , , , I , , . i I , , 0 5 1 0 1 5 2 0 2 5 3 0 TIME LAPSED AFTER PERCUSSION DRILLING OF SAMPLE (HOURS)

Figure 8

Experimentally determined decay curve (1) for2'4Pb and the normalized decay (2J for the same data.

0 S 10 IB 20

TIME BEFORE MEASURING GRADE OF ORE WITH SPECTROMETER (DAYS)

Figure 9

Correction curves for surficial uranium ore that has been partially degassed of 222Rn.

Naturally, it is not possible to analyse every sample in this manner because it is too time-consuming, but the lithologies chosen should be representative of the orebody to be used as future standard reference samples for the specific conditions and environment.

The next step is to normalize all the data of curve 1, such that the 0.72 kg eU/t becomes 1 kg eU/t and the new curve derived will have the form of curve 2 in Figure 8, which is similar to those of Figure 7. Extrapolation of the horizontal portion of the curve to time zero produces the "relative concentration of 222Rn in the uranium ore(%)"

which in this example is 30 %. Assuming that the lithologies chosen are fully representative of the orebody, each ore sample is assigned a radon value which will be characteristic of that type of sample. (In practice a mean value for each lithology should be used). Once these radon values have been established with a reasonably high degree of confidence, the grade estimation procedure during normal field operating conditions can proceed. The geologist is commonly hard-pressed to keep up with the logging of his percussion chip samples and usually he can log them only after a few days. If 3 d lapse before logging and the lithology in question has retained 30 % radon, as determined previously during the standardization procedure, then these two values are used to select the appropriate curve on Figure 9.1 n this case it is the 30 % curve for which the relative concentration of retained radon in the ore is 59.5 %. The equivalent grade of uranium in kg eU/t is then multiplied by a factor of 100/59.5 (or 1.68), which is a close approximation to the chemical uranium grade.

The alternative to this procedure is to seal the samples in a suitable gastight container and store them for 13 d or more before measuring the equivalent grade of uranium. This is demonstrated in Figure 9 where, after 13d the curves flatten out at 90% radon retention, which allows for a 10% experimental error.

4. CONCLUSION

The choice between the two techniques in (a) correcting for the induced disequilibrium, and (b) allowing the production of the radon gamma-emitting daughters before measuring the equivalent uranium, is determined by the necessity of evaluating the orebody on an on-going basis. If this is to be done soon after drilling, the radon compensation method must be applied. Care should be taken, however, because the method will not correct for natural disequilibrium inherent in the ore, for which only chemical analyses will provide reliable data.

This procedure for determining and compensatingforthe amount of induced disequilibrium would be particularly suitable for those situations where chemical analytical facilities are not readily available, and where on-going monitoring of the uranium grade on site is desirable.

REFERENCES

[1] DICKSON, B. L, Uranium series disequilibrium in the carnotite deposits of Western Australia, this Volume.

[2] HAMBLETON-JONES, B.B., HEARD, R., TOENS, P.D., Exploration forsurficial uranium deposits, this Volume.

A GEOSTATISTICAL EVALUATION OF THE NAPPERBY SURFICIAL URANIUM DEPOSIT,